Sorbent composition for an electrostatic precipitator

Abstract

A powdery calcium-magnesium compound used as a sorbent composition in flue gas treatment, compatible with electrostatic precipitators. The calcium magnesium compound is doped with calcium nitrate or nitric acid to reduce the electrical resistivity of the particles, increasing their collection efficiency.

Claims

1. A powdery calcium-magnesium compound comprising at least a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the powdery calcium-magnesium compound characterized in that it presents a resistivity at 300 C. (572 F.) (R300) lower than 1E11(110.sup.11) Ohms.Math.cm and higher than 1E7(110.sup.7) Ohms.Math.cm; and wherein the powdery calcium-magnesium compound is doped with calcium nitrate at an amount greater than or equal to 0.05 weight % and lower or equal to 5 weight % with respect to the total weight of the powdery calcium-magnesium compound.

2. A powdery calcium-magnesium compound, according to claim 1, further comprising a sodium based additive in an amount up to 3.5 weight % with respect to the total weight of the powdery calcium-magnesium compound, expressed as sodium equivalent.

3. A powdery calcium-magnesium compound according to claim 1, presenting a BET specific surface area by nitrogen adsorption of at least 20 m.sup.2/g.

4. A powdery calcium-magnesium compound according to claim 1, presenting a BJH pore volume for pores having a diameter lower or equal to 1000 by nitrogen desorption of at least 0.1 cm.sup.3/g.

5. A sorbent composition for a flue gas treatment installation including an electrostatic precipitator, the sorbent composition comprising a powdery calcium-magnesium compound having at least a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the powdery calcium-magnesium compound characterized in that it presents a resistivity at 300 C. (572 F.) (R300) lower than 1E11(110.sup.11) Ohms.Math.cm and higher than 1E7(110.sup.7) Ohms.Math.cm; wherein said powdery calcium-magnesium compound is doped with calcium nitrate and wherein said calcium nitrate is present at an amount greater than or equal to 0.05 weight % and lower or equal to 5 weight % with respect to the total weight of the powdery calcium-magnesium compound.

6. A sorbent composition according to claim 5 further comprising an additive selected from the group consisting of activated charcoal, lignite coke, halloysite, sepiolite, clays, bentonite, kaolin, vermiculite, fire clay, aerated cement dust, perlite, expanded clay, lime sandstone dust, trass dust, Yali rock dust, trass lime, fuller's earth, cement, calcium aluminate, sodium aluminate, calcium sulphide, organic sulphide, calcium sulfate, open-hearth coke, lignite dust, fly ash, and water glass.

7. A sorbent composition according to claim 5, comprising a sodium based additive in an amount up to 3.5 weight % with respect to the total weight of the powdery sorbent composition and expressed as sodium equivalent.

8. A sorbent composition according to claim 5, wherein said calcium-magnesium compound is hydrated lime.

9. A process for manufacturing a sorbent composition for a flue gas treatment installation including an electrostatic precipitator, the process comprising the steps of a) providing a powdery calcium-magnesium compound to a reactor, the powdery calcium-magnesium compound comprising at least a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the powdery calcium-magnesium compound characterized in that it presents a resistivity at 300 C. (572 F.) (R300) lower than 1E11(110.sup.11) Ohms.Math.cm and higher than 1E7(110.sup.7) Ohms.Math.cm; and b) adding an additive selected from the group consisting of calcium nitrate and nitric acid and combinations thereof in an amount calculated to obtain between 0.1 weight % and 5 weight % of calcium nitrate in weight of dry sorbent composition.

10. A process according to claim 9, wherein said calcium-magnesium compound comprises a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the dry calcium-magnesium compound.

11. A process according to claim 9, wherein said step of providing a calcium-magnesium compound to a reactor comprises a step of providing a quicklime to said reactor, slaking said quicklime with a predetermined amount of water to obtain said calcium-magnesium compound comprising at least a calcium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the dry calcium-magnesium compound with a predetermined amount of moisture.

12. A process for manufacturing a sorbent composition according to claim 9, characterized in that it comprises a step of adding a sodium based additive in an amount calculated to obtain up to 3.5% of sodium equivalent in weight of the dry sorbent composition.

13. A process for manufacturing a sorbent composition according to claim 11, characterized in that said step of slaking is performed in conditions such as to obtain hydrated time with a BET specific surface area measured by nitrogen adsorption of at least 20 m.sup.2/g.

14. A process for manufacturing a sorbent composition according to claim 11 characterized in that said step of slaking is performed in conditions such as to obtain hydrated lime with a BJH pore volume for pores having a diameter lower or equal to 1000 measured by nitrogen desorption of at least 0.1 cm.sup.3/g.

15. A process for manufacturing a sorbent composition according to claim 9, characterized in that it further comprises a step of adding an additional additive selected from the group consisting of activated charcoal, lignite coke, halloysite, sepiolite, clays, bentonite, kaolin, vermiculite, fire clay, aerated cement dust, perlite, expanded clay, lime sandstone dust, trass dust, Yali rock dust, trass lime, fuller's earth, cement, calcium aluminate, sodium aluminate, calcium sulphide, an organic sulphide, calcium sulfate, open-hearth coke, lignite dust, fly ash, and water glass.

16. A flue gas treatment process using an installation comprising an injection zone arranged upstream of an electrostatic precipitator, characterized in that it comprises a step of injecting in said injection zone a sorbent composition comprising a powdery calcium-magnesium compound having, at least a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the powdery calcium-magnesium compound characterized in that it presents a resistivity at 300 C. (572 F.) (R300) lower than 1E11(110.sup.11) Ohms.Math.cm and higher than 1E7(110.sup.7) Ohms.Math.cm and wherein said powdery calcium-magnesium compound is doped with calcium nitrate, said calcium nitrate being present at an amount greater than or equal to 0.05 weight % and lower or equal to 5 weight % with respect to the total weight of the powdery calcium-magnesium compound.

17. A flue gas treatment process according to claim 16, wherein said sorbent composition is injected in said injection zone wherein said flue gas has a temperature greater than or equal to 180 C. (356 F.).

18. A flue gas treatment process according to claim 16, wherein said sorbent composition is injected as a dry powder in a dry sorbent injection system or injected as an atomized slurry in a spray dryer absorber system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 presents a schematic embodiment of a flue gas treatment installation carrying out the flue gas treatment process with the sorbent composition according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) According to a first aspect, the present invention is related to a sorbent composition for flue gas treatment installation including an electrostatic precipitator, said sorbent composition comprising calcium-magnesium compound, characterized in that it further comprises an additive or a mixture of additives in an amount comprised between 0.1% and 5%, preferably 0.3% to 3% in weight of the dry composition, said additive or additives containing at least calcium nitrate.

(3) In a preferred embodiment, the calcium-magnesium compound is based on hydrated lime.

(4) Calcium hydroxide sorbents are manufactured by reacting (or slaking) calcium oxide, CaO or quick lime, with water in a so called hydrator, also called slaking unit. Alternatively, calcium magnesium hydroxide sorbents are manufactured by reacting dolomitic lime (also called dolime) or magnesium lime with water in a hydrator. Alternatively, quick lime and dolomitic lime can be mixed together and slaked with water in a hydrator to provide a mixture of calcium hydroxide and calcium magnesium hydroxide. In the following, the process of manufacturing of the sorbent composition will refer to quick lime but the process of manufacturing is not limited to quick lime as a starting material and dolomitic lime or a combination of dolomitic lime and/or magnesium lime and quick lime can also be used as starting materials.

(5) The process of manufacturing of the said sorbent composition according to the invention comprises a step of slaking quicklime with a predetermined amount of water to obtain hydrated lime with an predetermined amount of moisture, and is characterized in that it comprises a step of adding an additive or a mixture of additives to dope the sorbent composition in an amount calculated to obtain between 0.1% and 5%, preferably between 0.3 and 3.5% of said additive or mixture of additives in weight of the dry sorbent composition, said additive or additives containing at least calcium nitrate or nitric acid or a combination thereof.

(6) In an embodiment of the process of manufacturing the said sorbent composition, the predetermined amount of water in the said step of slaking is in a water to lime ratio 2:1 by weight or higher.

(7) In an embodiment of the process of manufacturing the said sorbent composition, the amount of water in the slaking step can be adapted to obtain a hydrated lime with a moisture less than or equal to 10 wt %, preferably less than or equal to 5 wt. %, preferably less than or equal to 2 w %, more preferably less than or equal to 1 w % with respect to the total weight of the sorbent composition at a powdery state.

(8) In another embodiment, the amount of water in the slaking step can be adapted to obtain a hydrated lime with a moisture content comprised between 5 wt. % and 20 wt. %. The amount of water in the slaking step can also be higher such as to obtain a hydrated lime with a moisture content above 20 wt. %, all % being expressed with respect to the total weight of the sorbent composition at a powdery state.

(9) In an embodiment, the hydrated lime obtained after the slaking step is dried in a further step.

(10) In an embodiment of the process of manufacturing of the sorbent composition according to the invention, the said additive containing calcium nitrate is used to dope the sorbent composition by adding the additive containing calcium nitrate as an aqueous solution or as a suspension or as a powder before or during the said step of slaking of calcium oxide or calcium magnesium oxide or a combination thereof.

(11) In another embodiment of the process of manufacturing of the sorbent composition according to the invention, calcium nitrate is added as aqueous solution or as a suspension or as a powder after the said step of slaking. Preferably, a step of drying is performed after the step of slaking and after the step of adding calcium nitrate. Calcium nitrate is preferably added to calcium hydroxide or calcium magnesium hydroxide before injection in an injection zone of the flue gas treatment installation.

(12) In a preferred embodiment of the process of manufacturing of the sorbent composition, the said step of slaking quicklime is performed in the conditions such as to obtain hydrated lime with a BET specific surface area from nitrogen adsorption of at least 20 m.sup.2/g and a BJH pore volume obtained from nitrogen desorption of at least 0.1 cm.sup.3/g. Various processes are available to the man skilled in the art to obtain an hydrated lime with such properties, and are disclosed for example in documents U.S. Pat. Nos. 6,322,769 and 7,744,678 of the applicant and incorporated by reference.

(13) In the process of manufacturing the sorbent composition according to the invention, particles of quicklime are advantageously used having a particle size distribution of less than 5 mm, in particular quicklime particles of particle size distribution 0-2 mm.

(14) Other processes for obtaining hydrated lime with high specific area and/or high pore volume can be found for example in U.S. Pat. No. 5,492,685 wherein an amount of alcohol such methanol or ethanol is added prior and/or the step of slaking quicklime and is removed after drying, in patent DE3620024 wherein sugar is added in the step of slaking for increasing the specific surface area and wherein glycols or amines are added to increase the flowability, in U.S. Pat. Nos. 5,277,837 and 5,705,141 wherein additives such as ethylene glycol, diethylene glycol, tri ethylene glycol, monoethanolamine, diethanolamine, triethanolamine or a combination thereof is added in the step of slaking for increasing the surface area of hydrated lime.

(15) In the process of manufacturing the sorbent composition, calcium nitrate can be added in certain amounts according to the invention as disclosed herein before the said step of slaking, during the step of slaking or after the step of slaking without substantially changing the BJH pore volume for pores having a diameter lower than or equal to 1000 of the sorbent composition. Moreover the BJH pore volume of the sorbent composition according to the present invention is substantially the same as for calcium hydroxide sorbent prepared by the known methods such as the one described in U.S. Pat. Nos. 6,322,769 and 7,744,678 incorporated by reference. Also, the BET specific surface area of the sorbent composition is above 20 m.sup.2/g. Therefore, the properties of the sorbent ensuring the efficiency of SO.sub.2 removal are preserved. Alternatively, nitric acid or calcium nitrate and nitric acid can be added before, during or after the step of slaking. Preferably, a higher BET specific surface area is obtained when calcium nitrate or nitric acid or a combination thereof is added after the step of slaking, and preferably before a drying step.

(16) In the said process of manufacturing the sorbent composition according to the invention, if a hydrated lime composition is prepared according to the method described in U.S. Pat. No. 7,744,678, such method comprises a step of adding a quantity of an alkali metal, preferably sodium in an quantity to the quicklime or to the slaking water or to the hydrated lime, sufficient to obtain in the hydrated lime an alkali metal content that is equal to or greater than 0.2% and equal or less than 3.5% by weight based on the total weight of the dry sorbent composition. The sodium can be added, for example, as Na.sub.2CO.sub.3. According to this embodiment, calcium nitrate or nitric acid or a combination thereof is further added after the step of slaking, and preferably before a drying step with an amount such as to obtain a content in calcium nitrate between 0.1% and 5%, preferably 0.3% to 3% in weight of the dry sorbent composition.

(17) Various sorbent compositions have been prepared according to the method of the present invention and measurements of the resistivity of dry powders of said sorbent compositions have been carried out in following the procedure outlined by IEEE (Estcourt, 1984). Basically, a resistivity cell of a determined volume is filled by a dry powder of sorbent composition and the powder is then compacted with a weight such as to obtain a flat surface. An electrode with a guard is placed over the surface of the powder and the resistivity of the powder is measured in an oven under a stream of air comprising 10% of humidity at various temperatures comprised between 150 C. (302 F.) and 300 C. (372 F.). The resistivity of comparatives examples have been measured in the same conditions. For each measurement, a maximum resistivity Rmax and a resistivity at 300 C. (572 F.) has been determined. The resistivity measurements are presented herein after:

Example set A

(18) Example 1 is a comparative sample of calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 6,322,769 B1. This sample was obtained from an industrial installation. No sodium based additive nor calcium nitrate nor nitric acid has been added.

(19) Example 2 is a comparative sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2. This sample has a content of Ca(OH).sub.2>90 w %, CaCO.sub.3<8 w %, and of Na.sub.2CO.sub.3 of about 0.8 w % and the rest of impurities. No further sodium based additive or calcium nitrate or nitric acid has been added. This sample was obtained from an industrial installation.

(20) Example 3 is another sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2 and wherein the lime comes from another source. This sample has a content of Ca(OH).sub.2>90 w %, of CaCO.sub.3<7 w %, and 2.1 w % of Na.sub.2CO.sub.3 and the rest of impurities. No further sodiurn based additive or calcium nitrate nor nitric acid has been added. This sample was obtained from an industrial installation.

(21) Example 4 is a calcium hydroxide sorbent manufactured according to the present invention using same source of lime as for the example 3 and using calcium nitrate as dopant in an amount of 1% relative to the dry product. This sample was obtained from an industrial installation.

(22) Example 5 is a calcium hydroxide sorbent manufactured according to the present invention using same source of lime as for the example 3 and using calcium nitrate as dopant in an amount of 2% relative to the dry product. This sample was obtained from an industrial installation.

(23) Example 6 is a calcium hydroxide sorbent manufactured according to the present invention, at laboratory scale by mixing (slaking), in a mixer with paddles, quicklime with stoichiometric amount of water and a quantity of Na.sub.2CO.sub.3 such as to obtain a sodium content of 2% by weight based on the total weight of the dried powdered composition obtained. The quicklime was obtained by calcination of lime from the same source of lime as for the example 3. After reaction in the mixer, the hydrated lime (calcium hydroxide) was discharged, dried and submitted to post treatment with 1% of HNO.sub.3 by weight of the dry product.

(24) Table 1 shows the measured resistivity parameters R.sub.max and R.sub.300 for those examples. All the measurements of resistivity parameters have been performed by measuring the resistivity of samples under increasing temperatures.

(25) TABLE-US-00001 TABLE 1 Resistivity parameters of calcium hydroxide sorbents of examples 1 to 6. Example R.sub.max ( .Math. cm) R.sub.300 ( .Math. cm) Ex. 1 8E12 3E12 Ex. 2 4E11 1E11 Ex. 3 9E10 4E09 Ex. 4 9E09 1E08 Ex. 5 6E09 4E07 Ex. 6 4E10 1E08

(26) From Table 1, it is clear that the both the R.sub.max value and the R300 value of Example 1 are high at and above the preferred range of resistivity values comprised between 10E7 ohms.Math.cm and 2E10 ohms.Math.cm. The presence of 0.8 wt. % of Na.sub.2CO.sub.3 in the sorbent composition of the Example 2 reduces the R.sub.max and R.sub.300 values by more than one order of magnitude respect to the R.sub.max and R.sub.300 values of the composition of example 1. The presence of 2.1 w % of Na.sub.2CO.sub.3 in the sorbent composition of example 3 reduces the R.sub.max and R.sub.300 values by more than two orders of magnitude respect to the R.sub.max and R.sub.300 values of the composition of example 1. Surprisingly the presence of a small amount of calcium nitrate in an amount of 1 wt % in the composition of example 4 reduces the R.sub.max value by nearly three order of magnitude and the R.sub.300 value by nearly four orders of magnitude respect to the R.sub.max and R.sub.300 values of the composition of example 1. The presence of 2 w % of calcium nitrate in the composition of example 5 decreases even more the values of R.sub.max and R.sub.300 relative to the composition of example 1. Therefore, surprisingly the addition of calcium nitrate or nitric acid is more effective for lowering the resistivity than the addition of a sodium based additive. Despite some differences due to the different process conditions (industrial scale and laboratory scale), the presence of calcium nitrate in the composition of example 6 by addition of HNO.sub.3 instead of by addition of calcium nitrate has the same tendency of the lowering the resistivity of the sorbent as the addition of Ca(NO.sub.3).sub.2.

Examples set B

(27) Example 7 is a sample of fly ash obtained from a coal power station.

(28) Example 8 is a blend of 80 w % of fly ash of example 7 with 20 w % of a sorbent according to example 3.

(29) Example 9 is a blend of 80 w % of fly ash of example 7 with 20 w % of a sorbent according to example 4.

(30) Example 10 is a blend of 80 w % of fly ash of example 7 with 20 w % of a sorbent according to example 5.

(31) Table 2 shows the measurement of resistivity parameters of Rmax and R300 for those examples 7 to 10. One set of measurements of Rmax and R300 has been performed by measuring the resistivity of the samples under increasing temperatures and one set of measurements of Rmax has been performed by measuring the resistivity of the samples under decreasing temperatures.

(32) TABLE-US-00002 TABLE 2 Rmax under R300 under Rmax under increasing increasing decreasing temperature temperature temperature ( .Math. cm) ( .Math. cm) ( .Math. cm) Example 7 3E10 5E09 3E10 Example 8 2E12 3E10 1E12 Example 9 1E11 1E09 2E10 Example 10 4E10 7E07 2E09

(33) The results presented in table 2 shows that for the same proportions of fly ash and calcium based sorbent, the blend of fly ash with a calcium based sorbent without calcium nitrate additive presents higher resistivity parameters Rmax and R300 than fly ash without calcium based sorbent, whereas the presence of only 1 w %, preferably 2 w % of CaNO.sub.3 additive in the calcium based sorbent has an positive influence on the resistivity parameters R.sub.max and R300 of the blend.

(34) It is to be mentioned that the examples of sorbent compositions presented herein above are not limitative for the present invention, and other additives in the amounts comprised between 0.1 and 5% in weight of the dry sorbent composition can be used to decrease the resistivity of sorbent compositions destined to be used in flue gas treatment processes using an electrostatic precipitator.

(35) It is to be mentioned that improvements of particulate matter collection on collecting electrodes of an electrostatic precipitators can be observed with the use of the sorbent according to the present invention.

(36) According to another aspect, the present invention is related to a flue gas treatment installation. FIG. 1 shows a schematic embodiment of a flue gas treatment installation 100 comprising an electrostatic precipitator 101 arranged downstream a first duct portion 102 arranged downstream an air preheater 103, characterized in that an injection zone 104 is arranged upstream said air preheater 103 and comprises a sorbent inlet 105. The said flue gas treatment installation 100 further comprises a reservoir 106 comprising said sorbent composition S to provide said sorbent composition to the said injection zone through the said sorbent inlet. The hot flue gas FG produced by a boiler 10 is flown through the injection zone wherein the sorbent S according to the invention is injected to react with SO.sub.2 and other acidic gases from the flue gas, then the hot flue gas crosses the air preheater through which cold air CA is flown to absorb the heat of the hot flue gas and to be injected as hot air HA in the boiler. Then the flue gas flows through the electrostatic precipitator 101 wherein charged collecting electrodes collects the particulate matter including the sorbent composition according to the invention that has reacted with undesired acidic gases. The flue gas treatment installation described herein is relatively simple and is well adapted for the use of the sorbent composition according to the present invention.

(37) Preferably the said flue gas treatment installation is used for treating flue gas of a power plant using coal or fuel containing sulfur species or other acid gas precursors.

(38) It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the appended claims.

(39) For example, in the preferred embodiment, the installation for flue gas treatment was described with an electrostatic precipitator downstream of an air preheater, said air preheater being connected to said electrostatic precipitator by a duct with an injection zone for injecting a sorbent composition according to the present invention arranged upstream of said air preheater. An alternative within the scope of the present may comprises a particulate collection device upstream of said preheater.

(40) Another alternative of the flue gas treatment device according to the present invention comprises in sequence an electrostatic precipitator, a preheater followed by optionally a particulate collection device, before reaching the chimney.

(41) The particulate collection device can be another electrostatic precipitator or any kind of filter, such as a bag house filter.

(42) In all of those embodiments, the sorbent composition according to the present invention is injected in an injection zone located upstream of said electrostatic precipitator, before or after the preheater, depending on the on-site configuration.